It is known that sulfate ion oxidation in an aqueous acid medium leads to the formation of peroxydisulfate ion; the main reactions are: at the anode EQU 2 SO.sub.4.sup.-- .fwdarw.S.sub.2 O.sub.8.sup.-- +2 e.sup.-
at the cathode EQU 2 H.sup.+ +2 e.sup.- .fwdarw.H.sub.2
The secondary reactions that disturb this phenomenon and reduce the current efficiency (or Faraday efficiency) are:
1st--electrolysis of the water which leads to the formation of oxygen at the anode and hydrogen at the cathode;
2nd--acid hydrolysis of the perosydisulfate ion into peroxydisulfate ion (or Caro's acid) ##EQU1##
3rd--reduction on the cathode of the S.sub.2 O.sub.8.sup.-- ion EQU S.sub.2 O.sub.8.sup.-- +2 e.sup.- .fwdarw.2 SO.sub.4.sup.--
It is known how to limit the first secondary reaction by using suitable anode materials, i.e. anode materials exhibiting the strongest oxygen excess pressure at zero current; platinum or platinum group metals such as ruthenium or metal oxides such as PbO.sub.2, RuO.sub.2, MnO.sub.2 are used. Addition of small amounts of compounds such as sulfocyanide ion, urea, etc., also makes it possible to restirct the first secondary reaction probably by modification of the adsorption properties of the platinum anode.
Control of the second secondary reaction can be assured by limiting the anolyte temperature to a sufficiently low value so that the hydrolysis rate will be slight, however, without greatly increasing the electric resistance of the electrolyte which would cause, with an equal current efficiency, a higher electric energy consumption.
To avoid the third secondary reaction, more or less satisfactory processes are used. In a first type process, the anode and cathode compartments are separated by a porous porcelain diaphragm which actually constitutes only a mechanical barrier that is hardly fluid-tight with regard to the persulfate ion, the cathode material used is lead; but as this metal is attacked in an oxidizing acid medium, in the case of continuous operation, it is necessary to operate with two circuits of liquid so that the cathode compartments are fed with persulfate-free aqueous solutions, which causes an efficiency loss.
In a second embodiment to control the third secondary reaction, lead is replaced by cylindrical graphite rod as the cathode and the cathode compartment, limited to the stationary phase, is confined in an asbestos band wound with joining spirals around the cathode. But the graphite has a tendency to split in the persulfate bath and the asbestos diaphragm hardens and becomes fragile. This splitting tendency of the graphite is greater in electrolytic preparations of sodium persulfate which--with this design of the cell--can be made with an optimal electrical efficiency only if the cathode surface is slight, and therefore if the cathode density is high; destruction of the cathode is then so rapid that this use is difficult to effect under economically acceptable conditions.
Recently, the life of cathodes has been greatly increased by using zirconium or a zirconium base alloy instead of graphite (in the absence of fluorine impurities, zirconium is completely unattackable in this medium) and polyvinyl chloride base synthetic materials, acrylic polymers or polyolefins instead of asbestos.
Use of zirconium makes possible the fourth embodiment in which use is made of a cell, without a diaphragm, made of a zirconium pipe forming the cathode and cell in which anodes, made of a conductive metal rod sheathed with platinum, are immersed; the useful volume of the cell is slight, on the order of 1 liter; the electrolyte circulates therein at high speed; the cathode density is high so that it is possible to ascribe a diaphragm role to the hydrogen film that is formed on its surface; this type of cell is that which leads to obtaining ammonium persulfate with minimal electrical energy consumption.
However, this embodiment has several drawbacks:
The gas mixture that is released at the top of the cell has a composition in the range of explodable H.sub.2 -O.sub.2 mixtures.
In electrolysis of ammonium bisulfate the current efficiency is greatly influenced by the persulfate concentration of the electrolyte and becomes almost zero for high concentrations; it is possible, to a certain extent, to improve this efficiency and bring it to acceptable values by using an imperfectly rectified current obtained from a single-phase alternating current and such that the rate of ripple of the rectified voltage is equal or close to 100%. This complicates the design of the rectified current generator and reduces its efficiency.
On the other hand, it is known that, with the cells described above, the formation of persulfate is influenced by the cation associated with bisulfate; current efficiency decreases in the direction of Cs.sup.+, K.sup.+, NH.sub.4.sup.+, Na.sup.+, Li.sup.+ ; for example, electrolytic preparation of sodium persulfate, with the cells described above, is not economically viable.